1. Field of the Invention
The present invention generally relates to methods for solubilizing boric acid and to boric acid solutions prepared by those methods. More specifically, the present invention is directed to methods particularly useful for preparing aqueous boric acid solutions containing about one pound boron per gallon of solution for use as fertilizer concentrates.
2. Description of the Background
Boric acid forms white, needlelike crystals in which the B(OH).sub.3 units are linked together by hydrogen bonds to form layers of nearly hexagonal symmetry. These boric acid crystals are not very soluble. In fact, the solubility of boric acid in cold water is only about 63.5 grams/liter at room temperature. Thus, saturated aqueous solutions of boric acid can contain no more than about one percent-by-weight boron.
As liquid compositions containing soluble boron have become increasingly popular as fertilizers, those skilled in the art have been interested in methods for increasing the solubility of boron. Because boric acid contains only about 18 percent-by-weight boron, the concentration of boron in solution is limited. However, those skilled in the art have been desirous of obtaining liquid solutions containing about 9-11 percent-by-weight boron.
Liquid boron solutions have long been prepared by dissolving an inorganic borate, e.g., sodium borate, in water. Because the borates are not significantly more soluble than boric acid, the maximum concentration of boron which can be achieved with saturated borate solutions is only about three percent-by-weight.
Other attempts to increase the concentration of boron in solution have involved the reaction of boron compounds, e.g., boric acid and the borates, with polyamines and alkanolamines to produce polyborates. Because these polyborates are more soluble in water, aqueous solutions containing higher boron concentrations may be prepared. Because it has been found that the boron in these polyborates is available to plants, polyborate solutions have found use as components of plant fertilizers.
Malouf and Docks have described the preparation of aqueous boron-containing compositions in U.S. Pat. No. 4,844,725. The aqueous boron-containing compositions disclosed by Malouf and Docks include the reaction product of boric acid and an alkylamine, together with an alkanol or alkylene glycol and water. These aqueous solutions have been used as boron sources for fertilizer.
In U.S. Pat. No. 4,332,609, Ott describes a liquid fertilizer composition containing a polyborate compound. The polyborate is prepared by reacting a boric acid compound with an alkanolamine or an aliphatic polyamine in a highly exothermic reaction. The solutions disclosed by Ott have been used for plant fertilization.
Howard described the preparation of another boron-containing composition for agricultural use in U.S. Pat. No. 4,572,733. The composition described by Howard is prepared by forming a solution of a borate in a water miscible glycol solvent, followed by addition of a ground, second boron compound to form a flowable composition containing a fine slurry of borate.
Downer described a spray oil containing boron for use as a foliar fertilizer in U.S. Pat. No. 4,155,739. These compositions are based upon borate esters and include amine-borate ester adducts.
U.S. Pat. No. 4,007,029 discloses liquid fertilizer compositions based upon anhydrous ammonia solutions including trace elements which may include boric acid.
Boron-containing solutions also have been used as wood preservatives and as cutting fluids. In U.S. Pat. No. 4,461,721, Gottsche describes a wood preservative comprising an aqueous paste containing boric acid, an organic amine and a polyborate. An aqueous cutting fluid containing the reaction product of boric acid with an alkanolamine, and preferrably including a long chain unsaturated fatty acid, is described by Sluhan in U.S. Pat. No. 2,999,064. Sluhan also describes cutting fluid compositions containing borate salts of alkanolamines or alkanoldiamines in his U.S. Pat. No. 3,186,946.
These and other patents relating to boron-containing solutions are summarized in U.S. Pat. No. 4,844,725 which is incorporated herein by reference.
While polyborates and boric acid/amine complexes are known, the specific methods and compositions of the present invention are unique. These methods and compositions permit the preparation of aqueous solutions characterized by high boron concentrations, while avoiding the highly exothermic reactions leading to the formation of polyborates. No known references have suggested, either directly or indirectly, the novel and non-obvious methods and solutions disclosed and clarimed herein. The novel and non-obvious methods and solutions disclosed and claimed herein have solved the long felt but unfulfilled need in the fertilizer industry for a simple method for preparing aqueous, high concentration boron solutions which are stable during long storage times at ambient temperatures. A further advantage of the methods and solutions disclosed and claimed herein is the inclusion of minor amounts of metal ions, particularly transition metals, which are beneficial for plant growth.